Regulated power supplies are universally used with electronic equipment for supplying DC power to such equipment at a voltage that is constant despite variations in load impedance and input power voltage. A regulated power supply senses the output voltage applied to the equipment and changes the operation of the power supply circuit in some manner to maintain the output voltage constant. Analog regulator circuits for power supplies are usually either the shunt or series variety. In the shunt variety, the impedance of a resistive element shunting the load is varied to maintain the voltage applied to the load substantially constant. In a series regulator circuit, a resistive element is placed in series with the load and the impedance of the element varies to maintain the output voltage constant. Both the series regulation approach and the shunt regulation approach are highly inefficient because they waste a great deal of power in the resistive shunt or series element.
As a result of the inefficiency of analog power supplies, switching power supply circuits have been devised which sequentially connect and disconnect the input line to the output, with the duty cycle or percentage of on-time being adjusted to maintain the output voltage substantially constant. The principle advantage of the switching power supply is that the solid-state switching elements connecting and disconnecting the load to the input are not operated in their linear region but instead operate at a low-voltage saturation point or a zero current cutoff point. Either of these points results in relatively little or no power dissipation in the switching element.
One commonly used switching power supply utilizes an "H-shaped" configuration in which two series-connected pairs of solid-state switches are arranged in parallel between two supply lines. The load is then connected to the interconnection between the switches of each leg, and the switches are operated in synchronism with each other so that the upper switch of one leg is closed while the lower switch of the other leg is closed. In this manner, current can be made to flow through the load in opposite directions even though the power supply is connected to a DC power source. The load receiving the AC signal can be a transformer in order to increase the supply voltage as desired. The signal from the transformer is then rectified and applied to a low-pass filter. The output voltage of the switching power supply can then be regulated by controlling the duty cycle of the solid-state switches.
Although the above-described switching power supply circuit has proved highly advantageous, primarily because of its high efficiency, it is apparent that a power supply requiring a large number of DC voltages of varying magnitude would result in a proliferation of circuitry utilizing the switching circuitry described above. Thus, there is a need to redundantly utilize the circuit elements of such power supplies wherever possible.
Another problem associated with switching power supplies arises from the internal capacitance inherent in solid-state switches. This capacitance allows current to flow through the switch even when the switch remains open. When the power supply is driving a load, this capacitive current flow does not present a problem because the duty cycles of the switches are automatically reduced to compensate for this current flow and thus maintain the output voltage constant. However, under no-load conditions, the capacitive current flow can increase the output voltage above the desired value. The duty cycle control circuitry for the switches cannot prevent this uncontrolled rise in output voltage since the switches are already open at all times, thus making further reductions in the duty cycle impossible.
Still another problem encountered with constant frequency, pulse-width modulated switching power supplies is their instability at duty cycles in excess of fifty percent.